Implement async get for direct actor call (#6339)

python/ray/experimental/async_api.py

@@ -88,9 +88,19 @@ def init():
     """
     assert ray.is_initialized(), "Please call ray.init before async_api.init"
 
+    # Noop when handler is set.
+    if handler is not None:
+        return
+
     loop = asyncio.get_event_loop()
     if loop.is_running():
-        asyncio.ensure_future(_async_init())
+        assert loop._thread_id != threading.get_ident(), (
+            "You are using async_api inside a running event loop. "
+            "Please call `await _async_init()` to initialize inside "
+            "asynchrounous context.")
+        # If the loop is runing outside current thread, we actually need
+        # to do this to make sure the context is initialized.
+        asyncio.run_coroutine_threadsafe(_async_init(), loop=loop)
     else:
         asyncio.get_event_loop().run_until_complete(_async_init())